When fine pixel size of a CMOS image sensor is obtained, there is a need to reduce the sizes of pixel transistors. However, if the sizes of the pixel transistors are reduced, then random noise (RN) gets worse.
In addition, when fine pixel size is obtained, in order to secure the areas of photodiodes (PDs), generally, element separation is formed by ion implantation. In this case, however, due to the formation of a gate electrode of a pixel transistor above element separation regions, white blemishes (white spots) increase.
Meanwhile, for a method for suppressing 1/f noise, there is devised a method for reducing the thickness of a gate oxide film of an amplification transistor (see, for example, Patent Document 1). However, if the thicknesses of gate oxide films above element separation regions are reduced, dark current gets worse, increasing white blemishes.
In addition, there is devised the formation of oxide films above element separation regions and above a part of an element region by a deposition method in order to block source/drain leakage caused by fixed charges in an enhancement type transistor when element separation is performed by a local oxidation of silicon (LOCOS) method (see, for example, Patent Document 2).